1. Field of the Invention
The present invention relates generally to the field of collimation and angle sensing structures. More specifically, the present invention is directed toward devices capable of detecting and measuring small changes in the propagation angle of an optical beam.
2. Discussion of the Related Art
In a conventional optical disc memory system, a beam of light from a light source is caused to converge on the surface of an optical disc by an objective lens. The surface of the optical disc has recorded thereon information in the form of depressions, pits, ridges, or other optically detectable changes in the reflective or transmissive properties of the disc. In a reflective type system, the beam is reflected from the disc in such a manner that it is modulated in accordance with the information recorded on the disc. The reflected laser beam is then directed onto the detecting face of a photodiode or other light detector which transforms the optical signal into an electrical signal. In this manner, the electrical signal carries the same information recorded on the disc and contained in the modulated light beam. This electrical signal is further processed, and ultimately results in the audio sounds or video images represented by the information recorded on the disc.
For accurate reading of the information stored on the disc, it is necessary that the beam of light be accurately focused on the recorded surface of the optical disc. When the light beam is focused on the disc, the information recorded thereon will be properly modulated onto the light beam.
Most of the methods that yield focus error signals rely on basic optical principles. In general, if the laser beam is in focus on the disc, the scanning spot reflected by the disc will be imaged back onto itself. For example, in one method, a collimated optical light beam is focused by an objective lens onto the optical disc such that a portion of the incident light beam is reflected from the disc back along the same optical path as the incident light beam. If the incident light beam is properly focused on the disc, then the reflected light beam will also be collimated after it passes through the objective lens in the reverse direction. By introducing some asymmetry into the light path of the reflected beam, it is possible to detect deviations from the optimum focus of the light beam on the disc. Many of such detection and measurement techniques utilize a lens or a prism to deflect or otherwise sample the reflected beam. The reflected light beam is then analyzed for characteristics indicative of focus errors.
In one class of methods, focus errors can be detected by measuring the degree to which the collimation of the reflected beam differs from the collimation of the incident beam. These measurements are often performed by detecting and measuring small angular deviations within the reflected light beam. Detecting and measuring small changes in the propagation angle of an optical beam may also be useful in many other fields. In optical memory systems, these techniques can be used to detect focal errors produced by changes in the distance between an objective lens and the media surface of an optical disc. Such focal errors are generally caused by warped or slightly eccentric discs. The objective lens is generally mounted on, and its position controlled by, a servo mechanism which moves along a direction parallel to the optical axis of the light beam, i.e., either increasing or decreasing the distance along the optical axis, between the objective lens and the optical disc. Movement of the objective lens with respect to the disc adjusts for any out-of-focusing conditions of the light beam on the surface of the disc which may occur. A focus error detection device is placed in the path of the reflected beam to detect focus sensitive changes and to produce correction signals which can be used as input to the servo mechanism.
Focus error detection systems have been previously proposed for controlling the movement of the objective lens with respect to the media surface of the optical disc. U.S. Pat. No. 4,691,098 to Maeda, for example, describes a focus control device in which an element, comprising alternating absorbing parts (which prevent the transmission of light) and transmitting parts (which allow light to pass through the structure), is positioned in the light path of the reflected beam. Each absorbing part forms a predetermined angle with respect to the optical axis, while each transmitting part is defined by its adjacent absorbing parts. A split detector is positioned behind the element and generates an output signal which is proportional to the difference of the quantity of light incident on each half of the detector. The quantity of light received by each half of the detector thus depends on the degree to which the absorbing parts of the optical element eclipse the incident light.
Ohsato, in U.S. Pat. No. 4,612,437 is directed toward a focus error detecting device which utilizes a compound lens, consisting of first and second lens regions, each having different focal lengths. The first region of the compound lens has a focal point which converges in front of a photodetector, while the second region of the compound lens converges to a point behind the photodetector. The photodetector is positioned midway between the focal points of the two lens regions. The photodetector comprises first and second photodetecting elements for receiving light which has passed through the first region of the compound lens, and third and fourth regions for receiving light which has passed through the second region of the compound lens. The difference between the sum of the signals from the first and fourth elements and the sum of the signals from the second and third elements produces a focus error signal. When the light beam is properly in focus on the disc, the difference of these sums will be zero. Conversely, if the light beam is not properly focused on the disc, one of the sums of the signals will be larger than the other, indicating the degree and direction of the focal error.
Many such previous methods have been found disadvantageous in that the focusing techniques require the beam to propagate over quite some distance before a measurement can be made. This distance requirement intrinsically puts a minimum limit on the size of the optical system. In addition, focusing systems that utilize prisms or lenses are typically larger than the reflected beam, occupying a large amount of space and increasing the weight of the system. Further, the manufacturing costs involved in producing optical components of at least moderate quality can be prohibitive.